1. Field of the Invention
The disclosed embodiments of the present invention relate to a calibration method, and more particularly, to a systematic defect analysis method which is able to calibrate mismatches between an in-phase signal path and a quadrature-phase signal path of a transmitter/receiver, and an associated apparatus.
2. Description of the Prior Art
A modulation technique which is defined as ‘complicated’ carries more information than in general cases. The data transmission rate may be increased through sophisticated modulation processes, e.g. 64-Quadrature Amplitude Modulation (QAM) or 256-QAM. A need for high level QAM is therefore increasing. To improve the transmission and receiving ability of the high level QAM, an Error Vector Magnitude (EVM) of a communication system has to be correspondingly improved. One factor affecting the EVM is the extent of an imbalance between in-phase and quadrature-phase (IQ imbalance). The main factor which induces IQ imbalance is a mismatch between the in-phase path and the quadrature-phase path of a radio frequency (RF) circuit. Even a slight deviation may result in an imperfect quadrature modulation/demodulation process and affect the overall communication system, which leads to a high reception bit error rate (BER). The deviation is divided into two categories: amplitude deviation and phase deviation, wherein both categories can introduce an image signal at symmetric frequency. FIG. 1 is a diagram illustrating a received signal at a receiving terminal and an introduced image signal of the received signal. The difference between the magnitude of the received signal and the magnitude of the image signal is known as the image rejection ratio (IRR). In practice, the IRR is inversely proportional to the extent of the IQ imbalance.
In order to solve the above issue, a calibration process (i.e. IQ calibration) may be performed upon a transceiver before the transceiver starts to transmit and receive data. The IQ mismatch phenomenon can present itself in two ways. A first way is, when sending carriers to a mixer of an in-phase path and a quadrature-phase path from a local oscillator (LO), it is difficult to keep a perfect phase difference of 90 degrees, or to guarantee that two amplitudes of the two signals are exactly the same. The other way is imperfection of the circuit manufacturing process, which causes component mismatches between the two paths, such as mismatches between a low-pass filter (LPF) pair, an analog-to-digital converter (ADC) pair, a digital-to-analog converter (DAC) or a gain amplifier pair. The component mismatches can also explain the interfering image signal as shown in FIG. 1 and are frequency dependent resulting in the extent of interference by image signals at different frequencies varying.
In summary, how to perform IQ calibration in a communication system has become an urgent issue in the field.